1. Field of the Invention
This invention relates to an imaging process. More specifically, this invention concerns an improved induction imaging process that is highly compatible with polar liquid development.
2. Description of the Prior Art
The formation and development of images on an imaging layer of photoconductive materials by electrostatic means is well known. The best known of the commercial processes, more commonly known as xerography, involves forming a latent electrostatic image on the imaging layer of an imaging member by first uniformly electrostatically charging the surface of the imaging layer in the dark and then exposing this electrostatically charged surface to a light and shadow image. The light struck areas of the imaging layer are thus rendered relatively conductive and the electrostatic charge selectively dissipated in these irradiated areas. After the photoconductor is exposed, the latent electrostatic image on this image bearing surface is rendered visible by development with charged finely-divided colored electroscopic powder material known in the art as "toner". This toner will be principally attracted to those areas on the image bearing surface having a polarity opposite to the change on said toner particles and thus form a visible powder image.
The development image can be read or permanently affixed to the photoconductor in the event the imaging layer is not to be reused. This latter practice is usually followed with respect to binder type photoconductive films where the photoconductive insulating layer is also an integral part of the finished copy, e.g. U.S. Pat. Nos. 3,121,006 and 3,121,007.
In so-called "plain paper" copying systems, the latent image can be developed on the imaging surface of a reusable photoconductor or transferred to another surface, such as a sheet of paper, and thereafter developed. When the latent image is developed on the imaging surface of a reusable photoconductor it is subsequently transferred to another substrate and then permanently affixed thereto. Any one of a variety of well known techniques can be used to permanently affix the toner image to the transfer sheet, including overcoating with transparent films and solvent or thermal fusion of the toner particles to the support of the substrate.
In the most popular of the xerographic systems of the type referred to above, the imaging member comprises a photoconductive insulating layer or amorphous selenium on a suitable conductive substrate. Due to problems inherent in preparation of such imaging members, and the increasing cost of materials used in said members, a number of alternative photoconductive imaging members have been proposed which eliminate such fabrication problems and reduce the amount of photoconductive materials, such as amorphous selenium, required by these members. In one such alternative photoconductor system, a photogenerator layer of amorphous selenium is laminated to a charge carrier transport layer of poly(N-vinylcarbazole), U.K. Pat. No. 1,337,228. The relative arrangement of selenium to poly(N-vinylcarbazole) vis-a-vis the conductive substrate is variable and determinative of the mode of sensitization of this member. However, since the relatively thin photogenerator layer of amorphous selenium is highly sensitive to abrasion it must be protected from the physical abuses occasioned during development and cleaning in order to insure reasonable photoreceptor life. It is thus preferred that the photogenerator layer be formed directly on the substrate and thereafter overcoated with the charge carrier transport layer. Although the materials proposed for use in such transport layers do have the ability to withstand greater abuse than the photogenerator layer due to their increased thickness and other inherent mechanical properties, the materials which have been proposed heretofore as suitable transport layers are highly sensitive to mechanical abrasion and damage, particularly under development conditions in which a development electrode is firmly pressed against the charged and imaged photoreceptor, and well as under cleaning conditions where a cleaning member may be scraped over the surface of the photoreceptor subsequent to image transfer. Such abrasion is almost unavoidable in an otherwise simple and efficient development process called polar liquid development, which is described in Gundlach U.S. Pat. No. 3,084,043. In this process, a polar developer or "ink" may be presented to the electrostatic latent image, e.g. by pressure contact with a serrated metal roller, such as a trihelicoid gravure coating roller. After image development and transfer, residual ink may be scraped off the photoreceptor surface by an absorbent sponge or web or by absorption in powder followed by web cleaning. For the photoreceptor to have mechanical durability under these conditions, and to protect the active photoconductive layer or layers against chemical damage due to corona ions, oxygen, and toner residues, it is highly advantageous to use a photoreceptor structure having a relatively tough abrasion-resistant protective surface layer comprising a dielectric polymer substantially devoid of photosensitivity to light in the visible and near ultraviolet region of the electromagnetic spectrum.
Surface protected photoreceptors having a dielectric overcoating with a dielectric thickness comparable to that of the photoreceptor layer have been used previously, as described by Hall (U.S. Pat. No. 3,234,019), by Mitsui (IEEE Trans. on Elect. Devices, vol. ED 19 No. 4, 1972 pp 396 - 404) and by Nakamura (ibid. pp. 405 - 412). The processes described for the use of such photoreceptors, however, are complex and restrictive, requiring multiple charging and illumination steps, and forcing the user to restrict the imaging process to either a negative-to-positive mode or to carry out image exposure during simultaneous corona charging. The latter restriction makes it impractical, for example, to make full frame or full frame flash exposures and thus restricts imaging, in effect, to the relatively inefficient slit-scan mode. As is well known, slit scanning is very wasteful of radiant energy. Given the limitations of photosensitivity of known photoreceptor materials, it has been impossible heretofore to produce a photoreceptor suitable for fast cyclic xerographic duplicating using full frame exposure in the positive-to-positive mode, with a photoreceptor having a thick, durable, and flexible protective plastic overcoating.
Accordingly, it is the object of this invention to remove the above as well as related deficiencies in the prior art.
More specifically, it is an object of this invention to provide an overcoated imaging member suitable for use in an induction imaging process which also has resistance to abrasion and oxidation.
It is another object of this invention to provide an induction imaging process compatible with the use of full frame exposure.
It is yet another object of this invention to provide an induction imaging process capable of preparation of multiple copies from a single image exposure.
It is another of the objects of this invention to provide an induction imaging process wherein the photoconductive insulating layer utilized in said process consists essentially of two distinct and separate phases; namely, a charge carrier generator layer and a charge carrier transport layer.
It is yet another object of this invention to provide an induction imaging process which is compatible with polar liquid development.
It is yet a further object of this invention to provide an induction type imaging process capable of rapid and repeated cycling.